Beef Stroganoff

Ingredients

• 2 pounds beef filet or flank
• 4 tablespoons butter or margarine, divided
• 1 onion, minced
• 1/2 pound fresh mushrooms, sliced
• 2 tablespoons flour
• 2 cups beef stock or bouillon
• 1/2 teaspoon paprika
• 1/2 teaspoon salt
• 1/4 teaspoon freshly ground pepper
• 1 cup sour cream, at room temperature